Supplementing Beef Cow Diets

Chip Poland, Area Extension Livestock Specialist
Dickinson Research Extension Center

Any nutritional program involves estimating dietary requirements, concentrations of required nutrients available in common feedstuffs and dry matter intake. When the supply of nutrients to an animal fails to satisfy requirements, supplements are often provided. Providing supplements help correct nutrient deficiency, increase pasture carrying capacity or stretch forage supplies, and provide a carrier for some type of feed additive.

Potential deficiencies can encompass energy, protein, vitamins and minerals. Although vitamin and mineral deficiencies can occur and result in an expensive loss of production, ensuring an adequate supply of these nutrients typically represents a small percentage of the overall costs of maintaining the cowherd. Performance of cattle consuming lower quality forages is often increased with supplemental energy and protein. An increase in digestible dry matter intake (DDMI) can explain most of the enhancement in animal performance. How this increase in DDMI occurs depends on the composition of the supplement, the quality of the basal forage, and the amount of the supplement consumed.

Most protein and energy supplements contain similar energy concentrations, so classification of supplements is typically based on their protein concentration. Given a similar protein need, supplements with a relatively high protein concentration require less feed than supplements with a lower protein concentration. Since energy concentrations are similar and protein concentrations are lower, energy supplements (low protein) require more feed, and thus supply more energy, than protein supplements when supplied on an equivalent protein basis.

Providing small amounts of protein supplement to cattle consuming a low protein forage has been shown to increase DDMI from forage. Explanations for this include an increased rate and/or extent of forage digestion, increased gut fill, increased rate of passage, and increased efficiency of nutrient use. Cattle have two distinct requirements for protein. In addition to a physiological demand for protein by the animal, the microbial population in the rumen have specific requirements for protein. Providing supplemental protein to optimize rumen fermentation of forage tends to enhance forage digestibility and intake.

The protein requirement of a cow is met by two types of protein. The first type is microbial protein synthesized in the cow's rumen. The second type is dietary protein that has escaped ruminal degradation but is still available to the animal. The portion of feed protein that escapes the rumen (UIP) is commonly referred to as undegradable or bypass protein. The portion that is degraded in the rumen (DIP) is used to facilitate microbial fermentation and growth. It is this microbial action that allows cattle to effectively utilize large quantities of forage to meet their nutrient requirements. In situations where the protein requirement of a cow is not being met, one must first determine whether the microbial needs in the rumen are being met. Rumen microorganisms must have an adequate supply of DIP to effectively grow and digest dietary fiber. One rule of thumb is that DIP should be supplied at roughly 13% of dietary TDN. If this level of DIP is not being supplied, then a protein supplement high in DIP should be used to meet this requirement. Once the DIP requirement has been satisfied, and if the supply of microbial protein will not meet the cow's physiological protein requirement, supplemental UIP should be considered. Since supplying DIP tends to be cheaper than UIP, forage-based feeding programs should focus on optimizing microbial activity in the rumen. This approach to protein supplementation should reduce cattle feeding costs, while providing the essential nutrients necessary to ensure future productivity.

Since supplement cost and protein concentration are typically positively related, lower protein supplements and feed grains are used when the energy intake is severely deficient. An imbalance between energy demand and supply can occur due to excessive energy demands, poor forage quality, or a lack of forage availability. Despite the negative effects on forage digestion and intake that can result, energy supplementation of forage diets can increase animal performance due to an increase in DDMI, an increase in the efficiency of nutrient use or an increase in the flow of undigested feed and microbial protein out of the rumen.

Supplementation studies where various amounts of a supplement are included in a forage-based diet indicate that if protein and energy requirements can be met with a low level of supplementation, forage utilization is generally improved. As the amount of supplement offered increases above this low level, forage intake and digestibility begin to decline. It has been suggested that forage fiber digestion decreases when ruminal pH falls below 6.7. Thus if supplement (or starch) intake depressed ruminal pH, this could help explain reductions in forage fiber digestibility associated with energy supplementation.

Energy supplementation with a readily degradable fiber source has been suggested as an option for maintaining ruminal stability in regard to fiber digestion, ruminal pH and minimizing intake reductions associated with energy supplementation. Although data do exist to support the use of higher fiber over higher starch (e.g. feed grains) energy supplements, other data suggest that the livestock responses are inconsistent. It appears that when supplements, regardless of source, are fed to correct a severe deficiency in energy intake, a reduction in ruminal pH and forage intake can be expected. It is at more moderate levels of supplementation, where protein supply is adequate, that supplement composition may differentially affect forage intake and digestibility.

The effect a supplement will have on a base forage depends on the level of supplemental intake, composition of supplement and the quality of the forage. In situations where forage is in short supply or quality is low compared to animal requirements, it may be more economical to discount the energy value of the base forage and use traditional energy supplements (e.g. feed grains) to provide the additional energy required. This assumes that the protein needs of both the rumen microbes and the animal proper are being met. Using traditional methods to calculate energy supply, and not correcting for interactive effects between forages and supplements by adjusting energy value and intake of the forage, may result in over- or under-estimation of energy supply. Either consequence will reduce the economic efficiency of feeding the beef cowherd.

February 1997